The present invention relates to a method of treating zirconium based metal waste particularly, though not exclusively, waste resulting from nuclear fuel reprocessing.
Fuel rods for nuclear plants comprise a core of enriched uranium material having an outer can or cladding of a zirconium based alloy. Presently, when the spent fuel rods are reprocessed, they are chopped up into shorter lengths and treated with nitric acid to dissolve out the spent fuel core, leaving behind the cladding since it is not attacked by the nitric acid. The pieces of zirconium alloy constitute so-called intermediate level waste which needs to be contained and stored safely for many years. One current method of dealing with this waste is to crush the pieces and store it encapsulated as the metal in concrete grout in drums.
A further problem with reprocessing irradiated fuel is that associated with isolating and dealing with the fission products generated during the nuclear reaction process. Normally, the fission products are separated from the uranium and plutonium, the latter two elements being reprocessed for further use. However, it is necessary to contain and safely store the fission by-products as they constitute so-called high-level waste. One method of dealing with this waste is by encapsulation by vitrification. Dealing with the zirconium waste and the fission product waste currently constitutes two separate stages of the reprocessing cycle and are both extremely costly in both plant and in running costs.
It is an object of the present invention to provide a process for dealing more economically with zirconium waste. It is a further object to provide an alternative and more cost effective means of dealing with and storing the fission product waste.
The present invention relates to a process for treating zirconuim based metal waste, the process including the steps of converting at least some of said zirconium based metal into an oxide (as herein defined). As hereafter described in more detail, the oxide is used in the production of a green body, for example by pressing, and the green body is sintered.
According to a first aspect of the present invention there is provided a process for treating zirconium based metal waste, said waste comprising at least some of the zirconium based metal in solution, the process including the steps of converting at least said solution of said metal into an oxide of said zirconium based metal; and, sintering said oxide to form solid articles.
One zirconium based mental alloy currently is use is known as "Zircalloy" (trade name) and comprises in excess of 95 wt % zirconium.
The step of bringing the zirconium based metal into solution by chemical or electrochemical means is known in the prior art and provides a stable solution, e.g., a nitrate and oxide residues. See, for example, "Use of Electrochemical Processes in Aqueous Reprocessing of Nuclear Fuels" by F. Baumgarter and H. Schmeider, Radiochemical Acta, Vol. 25 pp 191-210 (1978).
In this specification the terms "zirconium oxide" and "oxide of zirconium" and similar terms are frequently used. The actual chemical compositions resulting from the processes described herein may not have chemical compositions which correspond exactly either to a pure zirconium oxide or to zirconia, ZrO.sub.2, since the sintered materials in question will contain impurities and/or intentionally added materials and contaminants which it is desired to encapsulate, and/or to stabilize the crystal phase and which may also modify the crystal structure. Examples of such stabilizing and modifying additions may include, for example, metal oxides such as yttria, Y.sub.2 O.sub.3 to stabilize the crystal phase of zirconium oxide. Furthermore, in embodiments to be described below, particles of zirconium oxide powder are embodied in a matrix also containing aluminum and/or silicon atoms. Therefore, any reference herein to "zirconium oxide" or similar terms are to be taken as generic terms encompassing the resulting matrix of the sintered product or intermediate material in all embodiments and variations of the invention described herein howsoever arrived at.
The zirconium based metal may constitute waste resulting from irradiated fuel rods from nuclear plants for example.
The zirconium based metal may be brought into solution by electrochemical dissolution wherein the metal is made anodic in a electrolyte or nitric acid so converting the metal to a nitrate. In this method, a substantial proportion, perhaps about 85% of the zirconium metal, is converted directly to the oxide which forms a sludge in the dissolution vessel. The remaining nitrate may be thermally treated to decompose the nitrate to the oxide in a known manner.
The resulting oxide may be separated, dried and milled to break down friable flakes if necessary; the resulting powder being pressed, cast or extruded for example into "green" compacts and sintered to solid bodies in known manner at temperatures up to about 1800.degree. C.
Those steps in the ceramics art normally associated with the pressing and sintering of refractory oxide materials may be employed as desired and include such steps as appropriate as mixing with resin binders and/or lubricating waxes and preliminary burn-off treatments to remove such resins and waxes for example prior to sintering. Such steps are described in standard texts such as "Enlargement and Compaction of Particulate Solids", Ed. Nayland G. Stanley-Wood, Butterworths & Co. Ltd. 1983, particularly chapters 7 and 11; "Principles of Powder Technology", Ed. Martin Rhodes, Wiley, 1994, chapter 10; and "Principles of Ceramic Processing", J S Reed, Wiley Inter-science 1995, chapters 12, 17, 20, 22, and 29.
In practicing the present invention, where the zirconium based metal constitutes the cladding of a nuclear fuel rod, the whole fuel rod, including the irradiated uranium fuel, is preferable brought into solution in nitric acid. Thus, the solution will contain nitrates of uranium, plutonium, zirconium and also the fission products in the spent fuel. The uranium and plutonium may then be separated from the solution by one of the known so called "PUREX" processes which are essentially solvent extraction techniques. See for example, "The Chemistry of the Purex Process" by J. Malvyn McKibben, Radiochinica Acta 36 (1984) 3-15. This results in the solution retaining the fission by-products which are normally treated as a separate waste product. Again, the resulting nitrates may be thermally treated to decompose and convert them to oxide, including those of at least some of the fission products.
The zirconium based metal may alternatively be brought into solution by a route other than one of the so-called "PUREX" processes. For example, the zirconium metal waste may by converted to ZrX, where "X" is a halide, using an intensified fluorination technique such as by a fluidised bed with hydrogen fluoride. Other fluorinating agents such as nitrofluor (NOF:3HF) may also be used. The zirconium halides thus prepared may be readily converted to oxides.
Alternatively, oxides of the fission products may be separately treated and subsequently blended with the zirconium oxide powder in a preferred proportion.
A major advantage of the latter option is that the fission products are effectively encapsulated in the resulting sintered zirconium oxide body and a separate treatment stage for the fission products is removed from the process with a consequently great cost saving. Zirconium oxide is a particularly stable ceramic and has the necessary chemical durability to allow it to form the matrix for encapsulation of the high-level waste fission products. Furthermore, the melting point of zirconium oxide is greatly in excess of glass which forms the matrix in current verification encapsulation processes. The sintered bodies may be stored in drums in concrete grout for example. A further advantage conferred by the nature of zirconium oxide compared with glass is that it may allow higher levels of fission product waste to be incorporated into the ceramic encapsulate than is achievable with glass.
The sintered zirconium oxide material may also by used to encapsulate some or all of the plutonium arising from the nuclear reaction process in the same manner as described with reference to the fission products above.
The irradiated fuel rods may be processed as complete units without prior dicing into shorter lengths so improving the efficiency and ease of automation of the process and also reducing the contamination attributable to the dicing or slicing process. This again improves the economics of the process as a complete plant dedicated to cutting and handling of the fuel rod pieces may be dispensed with.